One of the problems encountered in the treatment of automotive exhaust gas, and in particular, exhaust gas from lean burn engine such as diesel exhaust gas and lean burn gasoline engines, concerns the treatment of nitrous oxides contained therein. To this effect, many exhaust gas treatment systems employed in automotive vehicles running under lean burn conditions incorporate a combination of a NOx storage catalyst located upstream of an SCR catalyst. In particular, nitrogen oxide contained in an exhaust gas stream is stored at lower temperatures in the NOx storage catalyst, to be released at higher operation temperature at which an effective reduction thereof in the SCR unit may be achieved. To this extent, it is usually necessary to employ a means of injecting a reducing agent into the gas stream downstream of the NOx storage catalyst and before the SCR unit for enabling the SCR reaction of nitrogen oxide to nitrogen. For achieving an optimal SCR activity, i.e. for keeping emissions of nitrogen oxides and/or of reducing agents such as ammonia and/or urea as low as possible, numerous solutions have been proposed to coordinate the activities of the individual components of an NOx storage catalyst, a reducing agent injection means located downstream thereof, and an SCR unit for reaction of the reducing agent and the nitrogen oxide under ideal stoichiometry depending on the inlet temperature.
DE 100 11 612 A1, for example, discloses an exhaust gas treatment system for combustion engines containing a sequence of an NOx storage catalyst, a urea injection unit, and an SCR catalyst provided in the exhaust gas conduit, wherein a system of sensors regulate the injection of urea depending on the oxygen content of the exhaust gas. WO 2008/022751, on the other hand, discloses an exhaust gas treatment system having the same sequence of components, wherein the infection of ammonia upstream of the SCR unit is specifically controlled in dependency of the exhaust gas inlet temperature.
US 2008/0045405 discloses an exhaust gas treatment system for a diesel engine, wherein a sequence of a diesel oxidation catalyst, a catalyzed soot filter, and an SCR unit are provided in that order in the exhaust gas conduit. As an improvement to said arrangement of the components, EP 2 112 339 discloses the inversion of the sequence of the catalyzed soot filter and the diesel oxidation catalyst, as a result of which a better control of the NO2/NOx-ratio may be achieved for the subsequent reaction in the SCR unit.
In the exhaust gas systems for the treatment of nitrogen oxides, and in particular, for the treatment of nitrogen oxides in exhaust gas from lean burn systems at air to fuel ratios of 20 to 1 or higher, the problem exists that the coordination of the hydrocarbon (HC) oxidation activity and of the NO oxidation activities remains poor after aging (i.e. after a durability test). In particular, for gasoline lean burn engines usually temperatures of >900° C. at lean and rich exhaust gas conditions occur during driving at higher speeds. These conditions usually lead to a very strong sintering of Pt, Pd or Pt/Pd alloys. This sintering process is accompanied by a strong loss of catalytic activity for hydrocarbon oxidation and in particular NO oxidation at state of the art TWC converters. However a high catalytic activity for NO oxidation is required for optimal conversion of the resulting gas mixture in a downstream SCR unit.